Digital watermark-embedding apparatus, digital watermark-embedding method, and recording medium having recoded therein a program for performing the digital watermark-embedding method

ABSTRACT

An input image is divided into local regions, each of which is formed by a macro block. Embedment intensity is determined for each of the local regions using a calculated characteristic amount and a compression bit rate. Digital watermarks are embedded at the determined embedment intensity into the input image at each of the local regions thereof. The input image is encoded, thereby providing a compressed bit stream that has the digital watermarks embedded therein.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a digital watermark-embeddingapparatus, a digital watermark-embedding method, and a medium havingrecorded therein a program for practicing the digitalwatermark-embedding method. In particularly, the present inventionrelates to a method for regulating the intensity required to embeddigital watermarks into a digital image.

[0003] 2. Description of the Related Art

[0004] With digital content such as digitized audio and digitized videodata it is easy to provide a faithful reproduction of original content.Therefore, it is a recent issue of importance to protect the copyrightof the digital content. A “digital watermark” is used as one solution tothe issue.

[0005] The digital watermark is an art of embedding data into video datain such a manner as to prevent human beings from perceiving degradationin image quality, and of detecting the embedded data from such imagedata.

[0006] The digital watermarks embedded in the image data must properlybe detected from pictures that have been subjected to image processingsuch as image compression and noise addition. A degree to which thedigital watermarks withstand such detection after the image processingis herein called toughness-with-image-processing.

[0007] The image quality degradation and thetoughness-with-image-processing form a trade-off relationship, and arebalanced with one another by adjusting a value of embedment intensity.

[0008] For example, an increased value of the embedment intensityenhances the toughness-with-image-processing while increasingdegradation in image quality of an image having the digital watermarksembedded therein. Meanwhile, a decreased value of the embedmentintensity inhibits the image quality degradation, but reduces thetoughness-with-image-processing.

[0009] A prior art digital watermark-embedding method designed toautomatically regulate such a trade-off relationship has been taught asan illustration in published Japanese Patent Application Laid-Open No.11-346302. This method is now described in brief.

[0010]FIG. 4 is a block diagram, illustrating a prior art digitalwatermark-embedding system. In FIG. 4, a category-classifying unit 401calculates a characteristic amount of an input image, and thendetermines, from the calculated characteristic amount, a category underwhich the image falls. The category-classifying unit 401 feeds acategory index to an accumulation apparatus 402.

[0011] The accumulation apparatus 402 has a digital watermarkcharacteristic table for each of the category indexes, and selects aproper characteristic table in accordance with the category index sentfrom the category-classifying unit 401. The digital watermarkcharacteristic table describes a relationship between digital watermarkintensity, a degree of degradation in image quality, and an evaluationof toughness with attack (image processing).

[0012] A digital watermark intensity-calculating unit 403 feeds thedigital watermark intensity into the accumulation apparatus 402, andthen calculates an optimum digital watermark intensity using the degreeof degradation in image quality, and the evaluation of toughness, andconstraint information. The degree of degradation in image quality andthe evaluation of toughness are fed from the accumulation apparatus 402,while the constraint information is entered from a user. The digitalwatermark intensity-calculating unit 403 delivers the calculated optimumdigital watermark intensity to a digital watermark-embedding unit 404.

[0013] The digital watermark-embedding unit 404 converts embedded datainto digital watermarks, and then embeds the digital watermarks into theinput image at the optimum digital watermark intensity that is fed fromthe digital watermark intensity-calculating unit 403. As a result, adigital watermark-embedded image is produced.

[0014] (1) The prior art employs activity (a squared average value of ACfrequency components) of the entire image as a characteristic amount ofthe input image.

[0015] However, the input image is not always uniformly formed. Innearly all of the input images, more diversified, complicated regionsand less diversified, simple regions are scattered.

[0016] An excess degree of intensity, at which the digital watermarksare embedded into the image, renders digital watermark-caused imagequality deterioration more conspicuous. In particular, suchdeterioration in image quality becomes more pronounced at lessdiversified regions of the input image.

[0017] As a result, the prior art has a problem in that the digitalwatermarks are often embedded at an improper degree of intensity overlocal regions of the input image, with a consequential degradation inimage quality.

[0018] (2) The degree of degradation in image quality according to theprior art is a numeric value calculated in accordance with the inputimage and the digital watermark-embedded image that is obtainedimmediately after the digital watermarks are embedded into the image.

[0019] Now, assume that video data accumulated and saved in a recodingmedium such as a hard disk are transmitted and distributed through anetwork. In this instance, in order to reduce an amount of data, videodata-forming images are encoded in accordance with, e.g., a MPEG systemfor moving pictures, or otherwise, e.g., a JPEG system for staticimages.

[0020] As a result, a pre-encoded, digital watermark-embedded imageusually differs from a post-encoded, digital watermark-embedded image.

[0021] For example, in a digital watermark-embedded image that has beencompressed according to a low compression bit rate, compression-causedblock distortions are noticeably observed. Therefore, such a digitalwatermark-embedded image is considerably poorer in image quality than apre-compressed, digital watermark-embedded image.

[0022] In short, the prior art takes no account of a change in imagequality between pre-compression and post-compression. This causes afurther problem in that the digital watermarks cannot be embedded intothe input image at a preferable degree of embedment intensity, with aconsequentially improper relationship between degradation in imagequality and the toughness-with-image-processing.

OBJECTS AND SUMMARY OF THE INVENTION

[0023] A first object of the present invention is to provide an art ofinhibiting degradation in image quality.

[0024] A second object of the present invention is to provide an art ofembedding digital watermarks into an input image at a proper degree ofembedment intensity when the image is compressed.

[0025] A first aspect of the present invention provides a digitalwatermark-embedding apparatus comprising: a characteristicamount-calculating unit operable to calculate a characteristic amount ofa digital image; an embedment intensity-determining unit operable todetermine embedment intensity using the characteristic amount calculatedby the characteristic amount-calculating unit and a compression bit ratefor use in encoding; a digital watermark-embedding unit operable toembed a digital watermark into the digital image at the embedmentintensity determined by the embedment intensity-determining unit; and animage-encoding unit operable to encode, using the compression bit rate,the digital image having the digital watermark embedded therein by thedigital watermark-embedding unit, thereby providing a compressed bitstream.

[0026] This construction allows the intensity of the digital watermarksto be determined on the basis of the compression bit rate for use inencoding and the characteristic amount of the input image. Morespecifically, the digital watermark-embedding intensity can bedetermined in view of image quality of a post-image encoded, digitalwatermark-embedded image, and the digital watermarks can be embeddedinto the digital image in accordance with a degree of compression.

[0027] A second aspect of the present invention provides a digitalwatermark-embedding apparatus as defined in the first aspect of thepresent invention, wherein the characteristic amount-calculating unitcalculates a characteristic amount for each local region that forms partof the digital image.

[0028] This construction calculates a characteristic amount for each ofthe local regions of the input image. This feature adjusts embedmentintensity for each of the local regions, and can inhibit degradation inimage quality. More specifically, the digital watermarks are embeddedinto the input image for each of the local regions thereof at embedmentintensity properly adjusted to balance with the characteristic amount.At this time, the digital watermarks are embedded in accordance with astate of each of the local regions of the input image. As a result,image quality degradation can be suppressed, which otherwise would berendered conspicuous as a result of the embedment of the digitalwatermarks.

[0029] A third aspect of the present invention provides a digitalwatermark-embedding apparatus as defined in the first aspect of thepresent invention, wherein the embedment intensity-determining unitemploys one of a table having digital watermark-embedding intensitycorrelated with a characteristic amount calculated by the characteristicamount-calculating unit, a table having the digital watermark-embeddingintensity correlated with a compression bit rate for use in encodingthat the image-encoding unit performs, and a table having the digitalwatermark-embedding intensity correlated with both of the characteristicamount and the compression bit rate.

[0030] The use of the table allows the embedment intensity to beadjusted in accordance with the compression bit rate. More specifically,the digital watermarks are embedded into the digital image at embedmentintensity properly adjusted in view of image quality of a post-imageencoded, digital watermark-embedded image.

[0031] The above, and other objects, features and advantages of thepresent invention will become apparent from the following descriptionread in conjunction with the accompanying drawings, in which likereference numerals designate the same elements.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032]FIG. 1 is a block diagram, illustrating an exemplary digitalwatermark-embedding apparatus according to an embodiment of the presentinvention;

[0033]FIG. 2 is an exemplary flowchart, illustrating how the digitalwatermark-embedding apparatus is operated;

[0034]FIG. 3(a) is an exemplary illustration, showing a table related toa characteristic amount;

[0035]FIG. 3(b) is an exemplary illustration, showing a table related toa compression bit rate; and

[0036]FIG. 4 is a block diagram, illustrating a prior art digitalwatermark-embedding apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0037] An embodiment of the present invention will now be described withreference to the drawings. FIG. 1 is a block diagram, illustrating aconstruction of an exemplary digital watermark-embedding apparatusaccording to the present embodiment.

[0038] As illustrated in FIG. 1, the digital watermark-embeddingapparatus includes a characteristic amount-calculating unit 101, anembedment intensity-determining unit 102, a digital watermark-embeddingunit 103, and an image-encoding unit 104. The image-encoding unit 104according to the present embodiment corresponds to a processing unit forperforming predetermined signal processing of a digital image havingdigital watermarks embedded therein.

[0039] The following discusses with reference to FIGS. 2 and 3 the wayin which the digital watermarks are embedded. FIG. 2 is an exemplaryflowchart, illustrating how the digital watermarks-embedding apparatusis operated.

[0040] At step S201, the characteristic amount-calculating unit 101divides entered digital image into a plurality of localized regionalimages that are predefined on a pixel-by-pixel basis. As a result, acharacteristic amount is calculated for each local region.

[0041] The local region according to the present embodiment is a macroblock that consists of sixteen pixels-by-sixteen pixels. According tothe present embodiment, a characteristic amount for each of the localregions is an average value of luminance components.

[0042] Alternatively, the local region defined as a single macro blockas just discussed may be a plurality of macro blocks that form a line,or otherwise may be each object or anything processable according toimage encoding.

[0043] The local region defined by a plurality of line-forming macroblocks provides a reduced calculation amount, when compared with thelocal region defined by the single macro block. Accordingly, thisalternative is useful in case of insufficient system resources.

[0044] The characteristic amount defined as an average value ofluminance components may alternatively be a sum of differential absolutevalues of horizontally and vertically neighboring pixels. When the localregion consists of sixteen pixels-by sixteen pixels, and when each ofthe pixels has luminance data of d (i, j) (0≦i≦15, o≦j≦15), then sum“Sh” of differential absolute values of horizontally neighboring pixelsis defined by an expression that follows: $\begin{matrix}{{Sh} = {\sum\limits_{i = 0}^{15}{\sum\limits_{j = 0}^{14}{{{d\left( {i,j} \right)} - {d\left( {i,{j + 1}} \right)}}}}}} & \left\lbrack {{EX}\quad 1} \right\rbrack\end{matrix}$

[0045] Sum “Sv” of differential absolute values of verticallyneighboring pixels is defined by an expression that follows:$\begin{matrix}{{Sv} = {\sum\limits_{i = 0}^{14}{\sum\limits_{j = 0}^{15}{{{d\left( {i,j} \right)} - {d\left( {{i + 1},j} \right)}}}}}} & \left\lbrack {{EX}\quad 2} \right\rbrack\end{matrix}$

[0046] A calculated characteristic amount according to visualcharacteristics of human eyes renders image quality degradation lessperceptible. This feature aids in increasing embedment intensity. Thevarious characteristic amounts as described above are offered merely forpurposes of illustration, and the present invention may encompass theuse of other characteristic amounts.

[0047] The following discusses with reference to FIG. 3 the way in whichthe embedment intensity-determining unit 102 practices processing atstep 202. Table 1 as illustrated in FIG. 3(a) shows a correlationbetween a characteristic amount and embedment intensity. Table 2 asillustrated in FIG. 3(b) shows a correlation between a compression bitrate and the embedment intensity.

[0048] Table 1 is used to compare the characteristic amount calculatedby the characteristic amount-calculating unit 101 with predeterminedthreshold values Y1, Y2 (Y2<Y1). “intensity 1” is determined accordingto such a range of the characteristic amount. For example, assuming thatthe characteristic amount is greater than threshold value Y1, then “3”is outputted as “intensity 1”.

[0049] Table 2 is used to compare the compression bit rate employed bythe image-encoding unit 104 with predetermined threshold values B1, B2(B2<B1). “intensity 2” is determined according to such a range of thecompression bit rate. For example, assuming that the compression bitrate is smaller than threshold value B2, then “3” is outputted as“intensity 2”.

[0050] The embedment intensity-determining unit 102 adds “intensity 1”to “intensity 2”, thereby feeding a sum of them as embedment intensityinto the digital watermark-embedding unit 103. The embedment intensityaccording the present embodiment is 6 (=3+3).

[0051] Although a sum of “intensity 1” and “intensity 2” is theembedment intensity to be fed into the digital watermark-embedding unit103, the present invention is not limited thereto. For example, aproduct of “intensity 1” and “intensity 2” may be conveyed asalternative embedment intensity to the digital watermark-embedding unit103.

[0052] At step S203, the digital watermark-embedding unit 103 embedsdigital watermarks into the image at the local regions thereof (i.e.,macro blocks according to the present embodiment) at the embedmentintensity determined by the embedment intensity-determining unit 102.The digital watermark-embedding unit 103 sends the image having thedigital watermarks embedded therein at the local regions thereof to theimage-encoding unit 104.

[0053] In this way, a series of processing at the characteristicamount-calculating unit 101, the digital watermark-embedding unit 103,and the image-encoding unit 104 is executed in a parallel manner foreach of the macro blocks. This feature provides less waiting time andmore efficient processing.

[0054] At step S204, the image-encoding unit 104 encodes the digitalwatermark-embedded image in accordance with a predetermined compressionbit rate, thereby producing a compressed bit stream.

[0055] As described above in detail, the digital watermark-embeddingapparatus according to the present embodiment divides a digital imageinto a plurality of local regions that are predefined on apixel-by-pixel basis, and then calculates a characteristic amount foreach of the local regions. The digital watermark-embedding apparatusdetermines embedment intensity using one table having the embedmentintensity correlated with characteristic amounts and another tablehaving the embedment intensity correlated with predetermined compressionbit rates.

[0056] The above system calculates a characteristic amount for eachsmall region of an input image. This feature makes it feasible to adjustembedment intensity for each of the regions, thereby suppressingdegradation in image quality. The above system adjusts the embedmentintensity in accordance with the compression bit rate. This featuremakes it feasible to determine the embedment intensity in view of imagequality of a post-image encoded, digital watermark-embedded image.

[0057] Typically, the digital watermark-embedding apparatus according tothe present embodiment realizes functions using a storage unit (a ROM, aRAM, and a hard disk), which contains predetermined program data, and aCPU (central processing unit) for performing the predetermined programdata. The program data may be introduced through a recoding medium suchas a CD-ROM and a floppy disk.

[0058] According to the present invention, a characteristic amount iscalculated for each small region of an input image. As a result,embedment intensity is adjustable for each of the regions, anddegradation in image quality can be inhibited. In addition, theembedment intensity is adjusted in accordance with a table that has theembedment intensity correlated with the compression bit rate. As aresult, the embedment intensity can be determined in view of imagequality of a post-image encoded, digital watermark-embedded image.

[0059] The above features provide improved toughness with standard imageencoding such as MPEG. For example, when a digital watermark-embeddedimage is compressed according to a low compression bit rate (highcompression and low image quality), increased embedment intensityprovides improved toughness with compression. Furthermore, image qualitydegradation such as compression-caused block distortions is moreperceivable than embedment-caused image quality degradation, and theembedment-caused image quality degradation can be suppressedindependently of increased embedment intensity.

[0060] Conversely, when the digital watermark-embedded image iscompressed according to a high-compression bit rate (low compression andhigh-image quality), decreased embedment intensity can suppress theembedment-caused image quality degradation. In addition,compression-caused degradation is small, and the toughness withcompression can be retained, even with a reduction in embedmentintensity.

[0061] Having described preferred embodiments of the invention withreference to the accompanying drawings, it is to be understood that theinvention is not limited to those precise embodiments, and that variouschanges and modifications may be effected therein by one skilled in theart without departing from the scope or spirit of the invention asdefined in the appended claims.

What is claimed is:
 1. A digital watermark-embedding apparatus,comprising: a digital watermark-embedding unit operable to embed adigital watermark into a digital image; and a processing unit operableto perform predetermined signal processing of the digital image havingthe digital watermark embedded therein, wherein said digitalwatermark-embedding unit is operable to embed the digital watermark intothe digital image at an embedment intensity that is determined based onhow said processing unit performs the predetermined signal processing.2. A digital watermark-embedding apparatus as defined in claim 1,further comprising: a characteristic amount-calculating unit operable todetermine a characteristic amount of the digital image, wherein saiddigital watermark-embedding unit is operable to embed the digitalwatermark into the digital image at an embedment intensity that isdetermined based on how said processing unit performs the signalprocessing, and that is determined based on the characteristic amountcalculated by said characteristic amount-calculating unit.
 3. A digitalwatermark-embedding apparatus as defined in claim 2, wherein saidcharacteristic amount-calculating unit is operable to determinecharacteristic amounts for each local region, respectively, that formspart of the digital image, and wherein said digital watermark-embeddingunit is operable to embed the digital watermark at embedment intensitiesthat are determined for each of the local regions, respectively.
 4. Adigital watermark-embedding apparatus as defined in claim 1, whereinsaid processing unit comprises an image-encoding unit operable to encodethe digital image having the digital watermark embedded therein, andwherein said digital watermark-embedding unit is operable to embed thedigital watermark into the digital image at an embedment intensitydetermined based on a compression bit rate that said image-encoding unitprovides.
 5. A digital watermark-embedding apparatus as defined in claim3, wherein the local region is one of a macro block for use in encodingperformed by said image-encoding unit or a plurality of macro blocks foruse in encoding performed by said image-encoding unit.
 6. A digitalwatermark-embedding apparatus, comprising: a digital watermark-embeddingunit operable to embed a digital watermark into a digital image, whereinsaid digital watermark-embedding unit embeds the digital watermark atembedment intensities determined for each local region, respectively,that forms part of the digital image.
 7. A digital watermark-embeddingapparatus, comprising: a characteristic amount-calculating unit operableto calculate a characteristic amount of a digital image; an embedmentintensity-determining unit operable to determine an embedment intensityusing the characteristic amount calculated by said characteristicamount-calculating unit and a compression bit rate for use in encoding;a digital watermark-embedding unit operable to embed a digital watermarkinto the digital image at the embedment intensity determined by saidembedment intensity-determining unit; and an image-encoding unitoperable to encode, using the compression bit rate, the digital imagehaving the digital watermark embedded therein by said digitalwatermark-embedding unit, thereby providing a compressed bit stream. 8.A digital watermark-embedding apparatus as defined in claim 7, whereinsaid characteristic amount-calculating unit is operable to calculatecharacteristic amounts for each local region, respectively, that formspart of the digital image.
 9. A digital watermark-embedding apparatus asdefined in claim 8, wherein the characteristic amount is a sum of anaverage value of luminance components over the local regions and adifferential absolute value of horizontally and vertically neighboringpixels over the local regions.
 10. A digital watermark-embeddingapparatus as defined in claim 8, wherein the local region is one of amacro block for use in encoding performed by said image-encoding unitperforms or a plurality of macro blocks for use in encoding performed bysaid image-encoding unit.
 11. A digital watermark-embedding apparatus asdefined in claim 7, wherein said embedment intensity-determining unitemploys one of a table having digital watermark-embedding intensitycorrelated with a characteristic amount calculated by saidcharacteristic amount-calculating unit, a table having the digitalwatermark-embedding intensity correlated with the compression bit ratefor use in encoding performed by said image-encoding unit, and a tablehaving the digital watermark-embedding intensity correlated with both ofthe characteristic amount and the compression bit rate.
 12. A digitalwatermark-embedding apparatus as defined in claim 11, wherein the tableis set to permit the digital watermark to be embedded at reducedintensity when the compression bit rate is increased, but to allow thedigital watermark to be embedded at increased intensity when thecompression bit rate is reduced.
 13. A digital watermark-embeddingapparatus as defined in claim 7, wherein said digitalwatermark-embedding unit is operable to embed the digital watermark intothe digital image usually at different values of embedment intensity foreach local region that forms part of the digital image.
 14. A digitalwatermark-embedding method comprising: embedding a digital watermarkinto a digital image; and performing predetermined signal processing ofthe digital image having the digital watermark embedded therein, whereinthe digital watermark is embedded into the digital image at embedmentintensity that is determined based on how the predetermined signalprocessing is conducted.
 15. A digital watermark-embedding method asdefined in claim 14, further comprising: determining a characteristicamount of the digital image, wherein the digital watermark is embeddedinto the digital image at embedment intensity that is determined basedon how the predetermined signal processing is performed, and that isdetermined based on the determined characteristic amount.
 16. A digitalwatermark-embedding method as defined in claim 14, wherein acharacteristic amount is determined for each local region that formspart of the digital image, and wherein the digital watermark is embeddedat embedment intensities that are determined for each of the localregions, respectively.
 17. A digital watermark-embedding method asdefined in claim 14, wherein the predetermined signal processingincludes encoding the digital image having the digital watermarkembedded therein, and wherein the digital watermark is embedded into thedigital image at an embedment intensity that is determined based on acompression bit rate for use in the encoding.
 18. A digitalwatermark-embedding method as defined in claim 16, wherein the localregion is one of a macro block for use in the encoding or a plurality ofmacro blocks for use in the encoding.
 19. A digital watermark-embeddingmethod, comprising: embedding a digital watermark into a digital image,wherein the digital watermark is embedded at embedment intensitiesdetermined for each local region, respectively, that forms part of thedigital image.
 20. A digital watermark-embedding method, comprising:calculating a characteristic amount of a digital image; determining anembedment intensity using the calculated characteristic amount and acompression bit rate for use in encoding; embedding a digital watermarkinto the digital image at the determined embedment intensity; andencoding, using the compression bit rate, the digital image having thedigital watermark embedded therein, thereby providing a compressed bitstream.
 21. A digital watermark-embedding method as defined in claim 20,wherein a characteristic amounts are calculated for each local region,respectively, that forms part of the digital image.
 22. A digitalwatermark-embedding method as defined in claim 21, wherein thecharacteristic amount is a sum of an average value of luminancecomponents over the local regions and a differential absolute value ofhorizontally and vertically neighboring pixels over the local regions.23. A digital watermark-embedding method as defined in claim 21, whereinthe local region is one of a macro block for use in the encoding or aplurality of macro blocks for use in the encoding.
 24. A digitalwatermark-embedding method as defined in claim 20, wherein one of atable having digital watermark-embedding intensity correlated with thecalculated characteristic amount, a table having the digitalwatermark-embedding intensity correlated with the compression bit ratefor use in the encoding, and a table having the digitalwatermark-embedding intensity correlated with both of the calculatedcharacteristic amount and the compression bit rate is employed.
 25. Adigital watermark-embedding method as defined in claim 24, wherein thetable is set to permit the digital watermark to be embedded at reducedintensity when the compression bit rate is increased, but to allow thedigital watermark to be embedded at increased intensity when thecompression bit rate is reduced.
 26. A digital watermark-embeddingmethod as defined in claim 20, wherein the digital watermark is embeddedinto the digital image usually at different values of embedmentintensity for each local region that forms part of the digital image.27. A recording medium, comprising: a digital watermark-embeddingprogram stored on the recording medium and operable to permit a computerto load the digital watermark-embedding program, wherein the program isoperable to instruct the computer to: calculate a characteristic amountof a digital image; determine an embedment intensity using thecalculated characteristic amount and a compression bit rate for use inencoding; embed a digital watermark into the digital image at thedetermined embedment intensity, and encode, using the compression bitrate, the digital image having the digital watermark embedded therein,thereby providing a compressed bit stream.